In recent years, hydrogen energy has gained attention as a solution to the increasingly severe issue of global warming, with underway plans to develop hydrogen infrastructure using pipelines for transport. Pipelines are subjected to cyclic stress due to gradual fluctuations in the internal hydrogen gas pressure during operation. In pressurized hydrogen gas environment, the strength of materials decreases compared to that in air—a phenomenon known as hydrogen embrittlement—which can cause crack propagation even under small stresses. Therefore, it is essential to understand the effects of hydrogen embrittlement caused by high-pressure hydrogen gas in pipelines and incorporate this knowledge into strength design to ensure safety and potentially reduce costs.

However, loading patterns used in previous studies have been biased, either by continuously loading specimens until fracture, as in tensile and fracture toughness tests, or by repeatedly applying very small sinusoidal stresses compared to the yield stress, as in fatigue crack growth tests. This study conducted cyclic loading and constant load-holding tests using triangular and trapezoidal waveforms with high loads and low frequencies under a high-pressure hydrogen gas, with the goal of simulating hydrogen pipeline operation in a more realistic way. The test results showed that crack growth was greater in tests using triangular waveforms than in those using trapezoidal waveforms, which included a load-holding phase.

Further post-mortem analysis included fracture surface observation, three-dimensional measurement of the fracture surface, and FEM analysis to investigate the effects of cyclic loading and load-holding on crack propagation from a practical perspective. Additionally, the Crack Mouth Opening Displacement (CMOD) value, measured with a clip-on gauge, gradually increased during load-holding in high-pressure hydrogen gas. To verify the accuracy of the measurements, the drift of the clip-on gauge was checked by leaving the specimen unloaded in high-pressure hydrogen gas for an extended period. The series of experiments suggested the possibility of hydrogen-enhanced crack blunting ahead of the crack tip though further examination is necessary to confirm its validity.

Moreover, deformed microstructures in the vicinity of the crack tip has been investigated in more details using Electron Channeling Contrast Imaging (ECCI) on specimens subjected to constant load-holding tests in both air and hydrogen gas. Based on these tests and observations, the potential of creep induced crack evolution in materials under high-pressure hydrogen gas was also examined.